When inspecting objects by means of optical observation means, the illumination of the objects is very important for the results of the inspection. In general, it is desirable for shadowing caused by non-uniform illumination, for example during the inspection of the surface of round objects, to be avoided. Shadows may be incorrectly regarded by the optical observation means as dirt or irregularities, which is deleterious to the inspection.
A device of this type is known. WO 03/023455 has disclosed a device in which light sources are positioned obliquely above and next to a conveyor in order to provide uniform illumination of the objects over the entire region in which the observation means observe the objects. The known device may have a plurality of conveyors arranged next to one another, in which case light sources are arranged next to and obliquely above each conveyor. In this case, light sources of a conveyor also illuminate the adjacent conveyor. In the known device, the light sources have to be positioned accurately in order to obtain uniform illumination of the objects. Furthermore, sufficient light sources per conveyor have to be provided. Furthermore, this arrangement of the light sources next to the conveyor means that only one or two objects can be placed next to one another on a conveyor to achieve good illumination.
Another device of the type referred to in the preamble is known from FR 2 699 696. This known device can be used, for example, to inspect the ripeness of fruit. The device comprises a box which is positioned above a reference plane. The reference plane may be moveable, so that objects are transported through the box. A ring of a transparent material is arranged approximately halfway up the height of the box. A light source which radiates light into the ring through an opening in the box is arranged outside the box. The ring transmits the light and is responsible for uniform illumination of the object, which is located beneath it approximately in the centre. An optical observation means is arranged in the top of the box for observing objects. The optical observation means as it were look at the object through the central opening in the ring. This known device is designed to illuminate and observe one object each time and is therefore not suitable for the inspection of large numbers of objects.